gpio: bcm-kona: Fix return value of bcm_kona_gpio_probe()

Propagate the error code returned by devm_platform_ioremap_resource()
out of probe() instead of overwriting it.

Fixes: 72d8cb71 ("drivers: gpio: bcm-kona: use devm_platform_ioremap_resource()")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
[Bartosz: tweaked the commit message]
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>